maybe because you are really excited and you did the test right away. if the test is done too early and the level of hormone is insufficiently present in your urine then it wouldn't give you any good clear result.

maybe due to the test sensitivity you have. home pregnancy tests differs from the levels of hCG that they are able to make detect. if it happens that you got a faint positive then maybe the one that you are using isn't just too sensitive enough for a conclusive result.

that is why it is important to read LABELS. somehow, the test sensitivity is detailed on the box of the kit, side part which is "mIU". say for example, 20mIU pregnancy test that will be able to positively detect sooner than of 50mIU test.

just don't hold on much with the faint positive result you got, there are still second time. take it easy and don't think negative. maybe this was just a false alarm you can still do it after a couple of days.